When producing molded parts, in particularly when they are at least partially hollow, there are substantial problems involved in removing the parts from the mold, that is demolding. These problems are particularly aggravated when the molded part has lateral projections so that it must be produced in a multi-part mold.
This is a particularly problem in making a cap for a blood-drawing kit as described in my U.S. Pat. No. 4,449,539. Such a cap forms a stable connection between the top of a blood vial and a cannula-carrying holder. Such a stable connection is for example desired also when mounting a blood-drawing tube on the device. In such a drawing device a cap is screwed onto the vial, which is provided with a piston, and in turn the cannula-carrying holder is fitted to the cap. To this end the cap has a cylindrical extension whose end wall has a central hole closed by a pierceable and self-sealing membrane. The cannula extends from both sides of its holder and has points at both ends so that as the holder is axially fitted to the cap, the back point is pressed axially through the membrane of the cap. The front point is the actual needle that can be inserted into a vein.
To retain the holder on the cap, the cap is normally provided with a radially projecting cylindrical lug or tab and the holder has an L-shaped groove or slot. The lug moves along an axially extending leg of the groove as the holder is fitted to the cap, and at the end of its travel the holder is twisted to lock the lug in an angularly extending shorter leg of the groove, like a bayonet mount.
It is essential that the holder be solidly secured to the cap. The single mounting lug has, therefore, been found to be inadequate in that it can allow the holder to tip so it has been suggested to provide two or more such lugs for a multipoint connection. In general it is desired to have at least two diametrally opposite lugs to prevent tipping of the holder on the cap, and preferably three to prevent tipping in any direction.
The cap is, however, an item that is mass produced in very large quantities, as such caps are used in large quantities and are discarded after a single use, so it must be made very cheaply. Hence the caps are typically formed in a so-called multi-daylight mold. Such arrangements have a central fixed mold part having a pair of opposite sides each forming with a respective movable mold part a plurality of mold cavities. The single lug can easily be formed by a pocket in the cavity of the movable mold part so that, once the mold is opened, the finished part can be knocked out of the fixed mold half axially, once of course a core element has been withdrawn. Obviously it would be impossible to make a two- or three-lug cap in such a multi-daylight mold since it would be necessary to form some of the lugs in the fixed mold part, making it impossible to knock out the molded pieces without shearing off the lugs in the fixed mold part.